There are already known various constructions of fuel cell devices, most if not all of which include a multitude of individual fuel cells that are arranged in fuel cell groups or stacks. As is well known, each such fuel cell includes an anode, a cathode, and a quantity of electrolyte or another ion transfer or exchange medium being present at least between (and often also within) the anode and the cathode. Then, as hydrogen (or another gaseous fuel) and oxygen (as such or as an ingredient of air) are supplied to the anode and the cathode, respectively, an electrochemical reaction takes place in each of such individual fuel cells, resulting in the formation of water as the reaction product, with attendant creation of electrical potential difference between the anode and the cathode that is then utilized, ordinarily in conjunction with that created in the other fuel cells, to supply electric power to an external user device or circuit. At least the effective region of each anode and of each cathode has to have a porous structure to allow penetration of at least the gaseous fuel and the oxygen, respectively, therethrough from the respective gaseous medium supply side to the areas at which the usually catalytically promoted electrochemical reaction takes place in the presence of the electrolyte. When arranged in a fuel cell stack, the individual fuel cells are typically separated from one another by respective separator plates that are interposed between the fuel cells and are usually electrically conductive but, to the extent possible, impervious both to liquids and gases.
Experience especially with fuel cell devices employing acid electrolytes has shown that separator plates that have the required degree of fluid, and particularly gas, impermeability are very difficult to make and hence expensive. Moreover, even if such discrete separator plates were less expensive than they currently are, they still constitute additional components Of the fuel cell stack which have to be separately stored, handled and ultimately assembled with the other components into the fuel stack. This further adds to the already considerable cost of the fuel cell stack or device.
To avoid these problems, it has been previously proposed, for instance in the U.S. Pat. No. 4,505,992 to Dettling et al, issued on March 1985, and in the commonly assigned U.S. Pat. No. 4,929,517 issued on May 29, 1990 to Luoma et al, to dispense with such discrete or separator plates, albeit not with their function. As disclosed there, this is achieved by forming a laminated assembly including two porous electrolyte retention plates in area contact with one another all over one major surface of each of them, with a sealant material initially present as a sheet between the two aforementioned major surfaces being forced during the performance of the lamination process to penetrate to a predetermined depth into the pores of both of the porous electrolyte retention plates in such a manner as to ideally completely fill or plug the affected pores and thus prevent fluid flow through such pores between the electrolyte retention plates.
Obviously, the material to be used as the sealant has to be compatible with the environment in which it is being used. This means that it not only must be highly invulnerable to the various gaseous and liquid media with which it may come into contact during the operation of the fuel cell device, such as oxygen, hydrogen, phosphoric acid or the like, that is, not to react therewith or not to be dissolved thereby, but also must be capable of withstanding the temperatures and temperature changes encountered during the lamination process and later during the use of the fuel cell device without suffering damage that would destroy or compromise its function as a fluid impermeable barrier. This drastically limits the choice of such materials.
Up to now, it was believed that only the materials mentioned in the above patents, and more particularly tetrafluoroethylene (TFE), perfluoroalkoxy tetrafluoroethylene (PFA), chlorinated tetrafluoroethylene (CTFE) or fluorinated ethylene propylene (FEP) are capable of satisfying the rigorous demands on the sealant to be used for this purpose. Unfortunately, it has been established that the performance of even these rather high-quality materials leaves much to be desired. In searching for the cause of their less than satisfactory performance, it has been determined that it is attributable, at least in part, to the difference between the coefficient of thermal expansion of the respective one of the above materials and that of the porous electrolyte retention plate (usually graphite or other carbonaceous material or mixture containing such carbonaceous material) which in turn, resulted in high residual internal stresses in the laminated assembly as the latter cooled from the lamination temperature. These residual stresses are so high for all of the above materials that they cause microcracking of the assembly with resultant fluid leakage through the sealant region.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a plate-shaped laminated fuel cell assembly including two porous components and an internal fluid impermeable barrier, which assembly does not possess the disadvantages of the known assemblies of this kind.
Still another object of the present invention is to devise a method of providing the laminated assembly of the type here under consideration in such a manner as to assure that the sealant constituting the fluid impermeable barrier penetrates to the desired depth into each of the porous components.
A concomitant object of the present invention is to develop the method of the above kind in such a manner as to be relatively simple and inexpensive to perform and yet to obtain reliable results.